The present invention relates to a high voltage winding assembly with improved regulation. More particularly, the present invention relates to a high voltage winding assembly or tertiary winding assembly of a flyback transformer which provides an improved flat-topped or substantially rectangular shaped pulse output, sometimes referred to herein as a square wave, resulting in improved voltage regulation for varying load conditions.
The present invention is particularly useful in the high voltage flyback transformer of television receivers and similar uses which require the development of a high voltage from a pulse input with a relatively moderate load current. In television receivers, the desire for brighter pictures from color television receivers has placed greater demands on the cathode ray tube high voltage power supply. The desired increase in brightness may be achieved by using a higher cathode ray tube anode voltage and improving the regulation of this voltage. The higher voltage itself is theoretically relatively easy to produce by increasing the turns ratio. However, even increasing the number of turns creates problems with core saturation. The corresponding improvement in regulation presents an even more difficult problem. In addition, corona discharge problems arise when the number of turns in a single high voltage winding is increased.
In accordance with the present invention, it has been found that a substantially rectangular pulse output from the high voltage winding of the flyback transformer produces improved voltage regulation. However, the tuning of the high voltage winding assembly must be accomplished in a manner so as to avoid excessive ringing after the pulse. The excessive ringing would cause distortion in the horizontal sweep output and possibly in the video circuits. By tuning for predetermined resonant frequencies in the range of 4.5 to 7.5 times the fundamental frequency of the input pulse and by controlling the relative amplitude of the tuned frequency signals, a substantially rectangular output pulse may be generated with a minimum or acceptable level of ringing.
In prior art attempts to produce the large d-c voltages, by increasing the number of turns on the high voltage winding in a conventional manner, the bulk of the high voltage winding created large capacitances and inductances which allowed tuning only at the lower frequencies. Tuning to frequencies in the range of 4.5 to 7.5 times the frequency of the applied pulse was impossible. The frequency of the applied pulse referred to is the reciprocal of twice the pulse width of the applied pulse.
In the past, attempts to provide the high voltage output with some degree of regulation have resulted in the use of smaller high voltage windings which produce a fraction of the required output voltage, for example one third the required voltage. A diode-capacitive voltage multiplier was then used in conjunction with this smaller high voltage winding to produce the required high voltage. By using a smaller high voltage winding, a certain degree of tuning and pulse output shaping were achievable to produce some degree of regulation. However, diodes used in the voltage multiplier adversely affected regulation. Furthermore, the capacitors required in such voltage multipliers were relatively expensive and were subject to failure thereby reducing reliability. Furthermore, due to the large capacitive currents, diodes used in the multipliers were subject to large current surges and possible failure thereby adversely affecting reliability.
Slot wound high voltage transformers per se have been known in the prior art. For example, U.S. Pat. No. 3,573,694 -- Eugene K. Von Fange et al. assigned to the assignee herein discloses a slot wound transformer provided with progressively increasing inner diameters for the high voltage slots. U.S. Pat. No. 3,644,986 -- Jugal K. Verma, also assigned to the assignee herein, discloses a method of tuning a high voltage transformer by electrically connecting a tuning capacitor in parallel with the primary winding. However, the prior art does not teach a slot wound transformer in accordance with the principles of the present invention which may be tuned to provide a rectangular, square wave or flat-topped pulse output which results in improved high voltage regulation. The term square wave, flat-topped pulse or rectangular shaped pulse are used interchangeably throughout, with it being understood that the term square wave conventionally means a pulse having relatively steep sides and a relatively flat top with dimensions which may be rectangular.